1. Field of the Invention
This invention relates to a process for producing a ferromagnetic metal powder, and more specifically, to a process for producing a ferromagnetic metal powder having a hysteresis curve suitable for a magnetic recording material as a result of suppressing the distribution of the coercive force of the magnetic recording material by reducing the distribution of the particle sizes of the ferromagnetic metal powder.
2. Description of the Prior Art
A number of methods for producing ferromagnetic metal powders are known, such as the thermal decomposition of organic acid salts of ferromagnetic metals, followed by reduction with reducing gases (e.g., as disclosed in Japanese Patent Publications Nos. 11412/61, 22230/61, 8027/65, 14818/66, 22394/68, and 38417/72, and The Record of Electrical and Communication Engineering Conversazione Tohoku University, Vol. 33, No. 2, pages 57, (1964), the reduction of needle-like oxyhydroxides with or without other metals or needle-like iron oxides obtained therefrom (e.g., as disclosed in Japanese Patent Publications Nos. 3862/60, 20939/64 and 39477/72, British Patent No. 1,192,167, and U.S. Pat. Nos. 3,598,568 and 3,681,018), the evaporation of ferromagnetic metals in inert gases (e.g., as disclosed in Japanese Patent Publication 27718/72 and Ohyo Butsuri (Applied Physics), Vol. 40, No. 1, page 110, 1971), the decomposition of metal carbonyl compounds (.e.g., as disclosed in U.S. Pat. Nos. 2,983,997, 3,172,776, 3,200,007 and 3,228,882), the electrodeposition of ferromagnetic metal powders using a mercury cathode, followed by separating the powders from the mercury (e.g., as disclosed in Japanese Patent Publications No. 15525/64 and 8123/65, and U.S. Pat. No. 3,156,650), and the reduction of salts of ferromagnetic metals in solution (e.g., as disclosed in Japanese Patent Publications Nos. 20520/63, 26555/63, 20116/68, 41718/72, U.S. Pat. Nos. 3,206,338, 3,494,760, 3,567,525, 3,535,104, 3,607,218, 3,663,318, 3,661,556, 3,669,643, 3,672,867, 3,756,866 and German Patent Application (OLS) Nos. 2,132,430, 2,326,258, 2,326,261, etc.
The last method described above which involves reducing a salt of a ferromagnetic metal as a solution thereof produces a ferromagnetic powder having the following defects.
(1) The particle sizes of the powder are widely distributed.
(2) The ferromagnetic powder obtained has a high coercive force, but since the distribution of the coercive force is wide, the slope, i.e., steepness of increase, of the magnetization curve (B-H curve) is poor. This causes a reduction in the sensitivity or the recording density, etc. of magnetic tapes using the ferromagnetic powder obtained.
(3) The dispersibility of the powder in a binder tends to become poor. Thus, a large amount of noise is developed in a magnetic tape using the powder and the tape has a bad S/N (signal-to-noise) ratio.
Various methods have been suggested previously to overcome these defects, such as a method in which the reduction reaction of a salt of a ferromagnetic metal as a solution with a hydrophosphite ion is carried out while applying ultrasonic vibration or magnetic field in order to reduce the distribution of the particle sizes (e.g., as disclosed in Japanese Patent Publication No. 41718/72), a method involving the addition of a protein, or a method involving the addition of a carbohydrate (e.g., both methods as disclosed in U.S. Patent Application Ser. Nos. 498,337 filed Aug. 19, 1974, (Japanese Patent Application (OPI) Nos. 18345/75, 19667/75, and 41506/75) and 498,338, filed Aug. 19, 1974 (Japanese Patent Application (OPI) No. 41756/75)). None of these methods, however, have been found entirely satisfactory.